1. Field of the Invention
The present invention relates to a process for the preparation of diaryl carbonates by reaction of an aromatic hydroxyl compound (e.g. phenol) with carbon monoxide and oxygen in the presence of a catalyst, a cocatalyst, a quaternary salt and a base which is characterized in that water is continuously removed with the reaction gas by stripping.
2. Description of the Related Art
It is known to prepare organic carbonates by oxidative reaction of an aromatic hydroxyl compound with carbon monoxide in the presence of a noble metal catalyst and a cocatalyst (German Offenlegungsschrift 2 738 437). The noble metals proposed are the elements of group VIIIb, palladium being preferably used. During the reaction, this palladium(II) species is reduced to palladium(0) and oxidized back to palladium(II) by oxygen with the aid of a cocatalyst. The cocatalysts used can be various manganese salts or cobalt salts in different oxidation states. In addition to these cocatalysts, a base, a quaternary ammonium salt and a desiccant are used. The solvent used is preferably methylene chloride.
In addition to the use as solvent of methylene chloride which is highly volatile, can be converted in the event of fire into phosgene, requires high expense for safety measures and must be recovered in a cost-intensive manner, disadvantages of this process are long reaction times and the poor space-time yields associated therewith. However, for an industrial reaction, the unsatisfactory reproducibility proves to be a decisive disadvantage, since from batch to batch, with an identical procedure, highly variable results can be obtained, even complete failure of catalysis.
In EP 350 700, the use of various cobalt salts, preferably cobalt acetate, is proposed as cocatalyst. In addition to this cocatalyst, the use of considerable amounts of various quinones or hydroquinones as an electron transfer catalyst is proposed. Separating off the electron transfer catalyst from the reaction mixture requires considerable expenditure in this process. Moreover, hydroquinones represent an additional aromatic bifunctional hydroxyl compound which can be reacted in the same manner to give carbonates. Separating off the byproducts formed in this manner can only be achieved with great expense. Recovery of the electron transfer catalyst used is not possible. The formation of these byproducts considerably decreases the selectivity and thus the economic efficiency of this process. The problem of inadequate reproducibility is also not solved in this application.
In the process proposals mentioned, the addition of a molecular sieve to bind water is necessary. In the absence of a molecular sieve, the conversion rate achievable turns out markedly lower, since carbonate formed is hydrolysed by reaction water formed at the same time. However, the use of molecular sieves makes the process unattractive for industrial use, since, for an effective separation of the water from the liquid phase, large amounts of molecular sieve (100 to 500% excess) are required which must be regenerated at high expense. The use of carbon dioxide as desiccant is proposed in EP 450 442. For this purpose approximately 30 to 35% of carbon dioxide is added to the reaction gas which is composed of oxygen and carbon monoxide. Considerable amounts of quinones/hydroquinones are also added here as electron transfer catalyst. The problems occurring owing to the addition of electron transfer catalysts with respect to byproduct formation, poor selectivity and loss of catalyst are, as already described above, also serious disadvantages here. Furthermore, the use of carbon dioxide as desiccant does not constitute an advantage. This is made clear by two essential points:
Firstly, carbon dioxide is only poorly suitable for drying the reaction mixture. This follows clearly from comparison of the examples in EP 450 442 (desiccant carbon dioxide) and EP 350 700 (desiccant molecular sieve). Thus in the presence of the desiccant carbon dioxide after 5 hours of reaction time only about one third of the yield is achieved which is obtained in the same time with molecular sieve as desiccant. Because of its low efficacy, carbon dioxide must be added in large amounts (approximately 35% of the reaction gas). This leads to a great dilution of the reaction gas, as a result of which the space-time yields achievable are dramatically decreased. Finally, in the event of circulation of the reaction gas, as the industrial conversion of this process requires, accumulation of the carbon dioxide in the gas stream must be prevented by complex processes. This additionally requires a high expenditure in terms of apparatus and high costs associated therewith which make economical utilization of this process impossible just for this reason.
Secondly, carbon dioxide does not behave in an inert manner to the reaction components. Bases such as, for example, sodium hydroxide can react with carbon dioxide to form insoluble products. As a result the catalyst system can be deactivated to the extent that no carbonate formation occurs any longer. Simultaneous use of carbon dioxide as desiccant and base in the catalyst system is therefore impossible.
In JP 04/257 546, a process is described in which the reaction with carbon monoxide in the presence of a noble metal catalyst and a quaternary salt is carried out by continuous feed into a distillation column. The reaction water is continuously distilled off.
It is a disadvantage in this process that, in order to remove the reaction water, the procedure must be performed in a distillation column which, owing to its construction, makes possible only short residence times. The space-time yields achievable by this process are therefore, at only 17.8 g/lh, very low.
This process disadvantage is particularly serious against the background of the extremely high catalyst usage which is necessary for these space-time yields. Thus in Example 1 of the application, at a loading of 182 g of phenol per hour, in total 3 g/h of palladium compound and 14.4 g/h of quaternary ammonium salt are used. At a reported yield of 35 g of diphenyl carbonate per hour, in one hour only 16.3 catalyst cycles are therefore achieved. This means that to prepare one kilogram of diphenyl carbonate by this process, at least 30 g of pure palladium (corresponding to 90 g of palladium compound) are needed. This requires very high capital costs for the catalyst and, additionally, high expense for the recovery of the noble metal. Economic utilization of the process is thus impossible. The use of large amounts of halides at high temperatures of 150.degree. to 180.degree. C., as required in this process, leads to great corrosion problems which involve high expenditure in terms of apparatus. It is further known to those skilled in the art that under the reaction conditions quoted the iodide of the quaternary salt preferably used is not stable and is oxidized to iodine to a considerable extent. This leads to great losses of the quaternary salt and to the formation of byproducts which greatly impairs the selectivity and thus the economic efficiency of this process.
In JP 04/261 142, a process is described in which the procedure is followed as in JP 04/257 546, with the difference that additional reactors are mounted on the distillation column to increase the residence time. The abovementioned problems with corrosion, catalyst consumption and loss of quaternary salt and the side reactions associated therewith are also not solved in this application. The proposed equipment likewise brings no advantages. The residence time is increased by the additional reactors. However, the proposed construction leads to a considerable back-mixing within the equipment so that side reactions can proceed to an increased extent, as a result of which the selectivity decreases. Thus in illustrative Example 1 of JP 04/261 142, a selectivity of only 97% is achieved in comparison with 99% in the comparable Example 1 of JP 04/257 546. The space-time yields achievable by this process are at approximately 9 g/l h lower still than with JP 04/257 546. Effective removal of the reaction water is impossible owing to the additionally mounted reactors. For in the proposed procedure, the reaction water formed in the reactors during the reaction is only removed subsequently in the distillation column. Under the reaction conditions, the carbonate formed in the reactors is cleaved again hydrolytically, as a result of which only very low conversion rates are achievable. In no application mentioned is the problem of unsatisfactory reproducibility solved so that, overall, a process which can be converted to an industrial scale has not hitherto been available.
The object was therefore to find a process which permits performing the synthesis of aromatic carbonates with continuous removal of the resulting reaction water at a high space-time yield under economic, industrially realizable and reproducible conditions.